In recent years, there is a strong demand for high performance and compact electronic devices. Thus, battery materials such as a lithium ion secondary battery, which are an energy source for such devices, are also required to be compact and light weight and to have a high capacity and a high energy density. As a consequence, there have been made various studies and developments on batteries. In general, a lithium ion secondary battery has a construction having a positive pole comprising a metal oxide, a negative pole comprising a carbonaceous material, and a separator and an electrolyte liquid sandwiched between both the poles. Such a second battery having a high energy density is actually used but is now desired to be further improved.
To cope with this demand, an attempt has been made to apply a solid electrolyte, which is a new ion conductor to be substituted for the conventional electrolyte solution, to electrochemical devices such as totally solid primary batteries, secondary batteries, and capacitors. Thus, various polymer electrolytes using a polymer compound as an electrolyte are now being studied. Polymer electrolytes have features that they have a flexibility and can follow mechanical shocks and volume changes of the electrodes that occur as a result of ion-electron exchange reaction between the electrodes and the electrolyte. As such a polymer electrolyte, U.S. Pat. No. 4,303,748 proposes a solid electrolyte comprising a polyalkylene oxide in which an alkali metal salt or an alkaline earth metal salt is dissolved. This electrolyte has a problem that the working efficiency is poor because a long time is required to dissolve the above salt. Further, the electrolyte has problems that its ion conductivity is not sufficient and its contact resistance with the electrodes is high. Such an insufficient ion conductivity and a high contact resistance result in an insufficient current density during charging and discharging so that the electrolyte may be used only for limited applications and cannot be used for applications requiring a large current.
To overcome the defects of the above solid electrolyte, there have been proposed a multitude of solid electrolytes including an alkali metal salt or an alkaline earth metal salt dissolved in a polymer having a poly(meth)acrylate main chain into which polyalkylene glycol chains are introduced as side chains and/or crosslinking chains. As one example of such polymer electrodes, JP-B-Hei3-73081 discloses a solid electrolyte having an alkali metal salt or an alkaline earth metal salt dissolved in an acryloyl-modified polyalkylene oxide. This electrode still has problems that its ion conductivity is insufficient, and further, the mobility of cation components affecting the charging and discharging is low. Such an insufficient ion conductivity and a low cation component mobility result in limitation of usage of the electrode, as mentioned above. Further, there is caused an additional problem that the electrode deteriorates in charging and discharging cycles because undesirable side reactions occur by movement of the counter anion.
For the purpose of controlling movement of ions affecting charging and discharging in a polymer electrolyte containing as a main ingredient a ring-open polymerization product from an alkylene oxide derivative, JP-A-Hei11-54151 and JP-A-2001-55441 propose an electrolyte using a trifunctional boron compound such as a boroxin ring capable of capturing counter anion of the metal salt. As the boron-containing compound for use in obtaining these compounds, orthoboric acid or boron oxide is used. In this case, however, water is produced during the reaction. Further, the above compound thus obtained is easily hydrolyzed with water. Therefore, it is very difficult to remove the water produced by the reaction. For this reason, residual water in the compound thus obtained is unavoidable, which may cause troubles when used as an electrolyte substrate. JP-A-2001-72876 and JP-A-2001-72877 propose an electrolyte of a boron-containing compound and refer to borane as a base material for obtaining the compound. Borane, however, has very strong activity and exhibits spontaneous combustibility in the air so that it is difficult to handle borane for production of the boron-containing compound. Additionally, when borane is used for reaction with a polymerizable group-containing compound, there is a possibility that the polymerizable group is damaged thereby.
On the other hand, there has been a proposal to use a polymeric boric acid ester as an electrolyte. It is known that a boric acid ester compound is obtainable by reaction of an alcohol with boric acid or anhydrous boric acid. Namely, the reaction in the case of using an alcohol and boric acid is as shown in the formula [1] below, while the reaction in the case of using an alcohol and anhydrous boric acid is as shown in the formula [2] below:H3BO3+3ROH→B(OR)3+3H2O  [1]B2O3+3ROH→B(OR)3+H3BO3  [2]
Since the boric acid ester compound has extremely high hydrolyzable property, it produces boric acid and an alcohol upon contact with water according to the reverse reaction of the formula [1]. In fact, the reaction of the formula [1] is an equilibrium reaction. The equilibrium is, however, extremely partial to the left, i.e. toward the direction resulting in the hydrolysis of the boric acid ester and the formation of boric acid and alcohol. Therefore, with the normal operation, the yield of the boric acid ester is extremely low. In this circumstance, it is a general practice, in the reaction of an alcohol and boric acid, to use an azeotropic dehydrating agent such as benzene to successively remove water produced in situ by the reaction from the reaction liquid, so as to shift the equilibrium of the above formula rightward and to recover the end product. Even with this method, because the equilibrium of the formula [1] is extremely partial to the left, the dehydration efficiency becomes poor as the reactivity increases and, therefore, there is a limitation on reduction of the water content. In addition, there is a problem that it is necessary to reduce the contents of boric acid and alcohol in order to obtain a high purity boric acid ester.
That is, while the use of an alcohol in an excessive amount is advantageous from the standpoint of an increase in the reaction rate and dehydration efficiency, it is difficult to evaporate the alcohol when the alcohol has a high molecular weight or when the alcohol has a polymerizable group which is susceptible to undergo polymerization upon heating. Thus, the alcohol is apt to remain unremoved in the system. When such a compound having a hydroxyl group remains in the system, the performance of an electrochemical device is markedly deteriorated, though depending upon its use.
When boric acid is used in an excessive amount, there is obtained a mixture of the boric acid ester and boric acid. When this mixture is heated, the boric acid ester is decomposed so that the yield is reduced. To cope with this problem, JP-A-Hei3-74390 discloses a method in which boric oxide and -an aliphatic alcohol are reacted to obtain a reaction liquid containing a boric acid ester and boric acid, the boric acid is then separated by filtration from the reaction liquid and the filtrate is distilled. This publication indicates that, if distilled without the separation of boric acid by filtration, the decomposition of boric acid ester and boric acid is accelerated as described above, thereby lowering the yield of the end product. This method, however, is applicable only to the production of compounds such as boric acid ester compounds of aliphatic alcohols which permit the separation of boric acid by filtration and which permit the purification by distillation.
In contrast, with the method using anhydrous boric acid as shown in the formula [2], 50% of the boron supplied may be converted into a boric acid ester without using an azeotropic dehydrating agent. However, the remainder of the boric acid is esterified through the reaction similar to the formula [1]. Therefore, there still remains a limitation on obtaining a high purity boric acid ester as described above.
Thus, when such a boric acid ester containing a large amount of impurities is used as a raw material for an electrolyte, there is a problem of high possibility of causing deterioration of electrolyte characteristics such as an increase in resistance of solid electrolyte interface (SEI), a reduction of charging-discharging cycle performance, and a reduction of potential stability. In particular, as far as an electrolyte for lithium ion secondary batteries is concerned, there is a great tendency that impurities contained in the boric acid ester compound will react with lithium to generate a gas, thereby causing a problem of reduction of the safety of the batteries.
In this circumstance, there is a demand for high purity boric acid ester compounds which contain a reduced amount of impurities such as water and which are usable as an electrolyte for electrochemical devices, etc.